1. Field of the Invention
The invention relates to an intervertebral spacer useful for spinal surgery and, more particularly, to a porous, strong intervertebral spacer formed of a biologically inert material.
2. Description of the Prior Art
Techniques and devices for fusing two or more vertebrae of the spine together are well known. Such techniques are commonly performed to correct problems, such as chronic back pain, which result from degenerated intervertebral discs. One technique for fusing together two or more vertebrae of the lumbar spine includes excising a portion of the disc between adjacent vertebrae and inserting one or more portions of an intervertebral spacer of a desired shape between the adjacent vertebrae. The intervertebral spacer may be inserted by either an anterior or posterior approach to the spinal column depending on a number of factors, including the number of vertebrae to be fused and past operative procedures. Upon healing, the vertebrae are desirably fused together through the intervertebral spacer.
Intervertebral spacers have been described by a number of names, including spinal implants and spinal cages. For convenience, all such devices will be referred to herein as xe2x80x9cintervertebral spacers.xe2x80x9d
Conventionally, intervertebral spacers have been autogenic bone harvested from other areas of the body, such as the pelvis, allogenic bone taken from cadavers or xenogenic bone, such as bovine bone sections. However, the use of bone grafts can add complications to the fusion procedure. For example, when using an autogenic bone graft, a second incision must be made in the patient to harvest the additional bone to be used in the graft, thus increasing the pain and blood loss to the patient. When allogenic or xenogenic bone grafts are used there is a potential for the transmission of disease from the cadaver or other graft source to the patient.
The use of non-biological implants, such as carbon fiber spacers, also has been attempted in the past, but these spacers tend to lack sufficient porosity and tissue ingrowth characteristics to function adequately. It would be desirable to provide a non-biological spacer which is non-reactive in the body and which has the strength and tissue ingrowth characteristics of a bone graft spacer.
In view of the aforementioned needs, the present invention provides a new and improved porous intervertebral spacer that can be used in the same manner as a bone graft spacer to fuse vertebrae together. The Titanium Bead Spacer Patent discloses and claims a porous intervertebral spacer composed of titanium beads formed by sintering the beads together into a porous shape in a mold of a desired shape and size. The Cage Plate Patent discloses a plate to which a cage, or spacer, is attached. The spacer is composed of a variety of materials that are worked on in a variety of ways. The present invention relates to a porous intervertebral spacer per se, including materials from which the spacer can be made and various techniques for forming the spacer.
In general, the intervertebral spacer according to the invention is made of a biocompatible material that has enough strength to adequately support adjacent vertebral bodies and that is porous enough to permit tissue ingrowth and bony fusion. The spacer comprises a rigid, porous body that includes a plurality of randomly sized, substantially interconnected voids that provide porosity throughout the body. Desirably, the spacer contains solid material within the range of about 45 to 75 percent of the total volume of the spacer; thus, the spacer has a porosity within the range of 25 to 55 percent.
In accordance with one technique for manufacturing the spacer according to the invention, the spacer is comprised of polymer pellets. The pellets are fused together in a mold of a desired shape. The size of the pellets determines the porosity of the finished spacer. In certain applications it may be desirable to mix pellets of various sizes to obtain a finished spacer having a variable porosity. Desirably the pellets are spherical beads made of PEEK (polyaryl, ether, ether ketone) resin that occupy a range of 45 to 75 percent of the volume of the spacer.
In another technique, the spacer is made of a plurality of strands of a biologically inert material. A porous metallic fiber mesh is formed by interengaging and intertwining the strands, which then are sintered together into a porous, rigid shape.
In accordance with another technique, the spacer is comprised of pellets that are intermixed with a plurality of strands of a biologically inert material. The pellets and the strands are sized such that they yield, when fused, a spacer with the fused pellets and strands occupying a range of 45 to 75 percent of the volume of the spacer. Strands of wire mesh, preferably titanium or titanium alloy, are intermixed with titanium or titanium alloy pellets to form a spacer having variable qualities of strength and porosity. The strands and the pellets also can be made of other biocompatible metals or a strong, biologically inert polymer such as PEEK.
In accordance with yet another technique, the spacer is comprised of void-containing foam metal. In this technique, the spacer is formed by mixing a blowing agent with powdered metal, heating the mixture to foaming temperature, and cooling the metal. The resulting product contains numerous interconnected pores of different sizes and shapes.
In accordance with yet another technique, the spacer is comprised of void-containing powdered metal. In this technique, the spacer is formed by filling a mold with a uniform mixture of (1) beads made of wax or other suitable low melting temperature material, (2) powdered metal, and (3) a binder. The mold is heated to a low temperature to set the binder and melt the beads and thereby remove them. Thereafter, the mold is heated to a high, sintering temperature to oxidize the binder and solidify the powder into a porous shape.
In accordance with yet another technique, the spacer is comprised of void-containing ceramic materials such as alumina or silica or combinations of ceramic materials. The spacer can be formed as described previously, for example, by mixing ceramic powders with foaming agents and heating the mixture to a temperature adequate to form interconnected voids and sinter the ceramic ingredients.
Yet an additional technique for forming the spacer according to the invention is to take a solid block of a biologically inert, strong material such as PEEK polymer, titanium, or ceramic, the block being shaped as desired by the surgeon, and to drill or otherwise form a series of openings or bores in the block. Such openings or bores could be formed by EDM, chemical attack, or any other known machining technique. Preferably, the openings or bores are variably sized and variably spaced, and will intersect at numerous, randomly located places within the spacer so as to permit and promote tissue ingrowth and bony fusion.
In all of the other embodiments disclosed herein, the spacer can be formed in a variety of shapes such as a prism (for example, a rectangular prism), a cylinder, or a plate. A particularly desirable shape is a body defined by spaced, parallel, top and bottom faces, the top and bottom faces being of the same size and shape; a pair of spaced, parallel side walls; a first, flat, end wall; and a second, curved end wall, the second end wall being curved along a radius extending between the spaced side walls, the radius approximating the anterior portion of a vertebral body. The body of the spacer can be provided with through bores and external ribs, fins, or notches for various purposes.
The foregoing and other features and advantages of the invention are fully described hereinafter. The accompanying drawings constitute a part of the specification and illustrate an exemplary embodiment of the invention.